1. Field
The present invention relates to the field of telecommunications; and more particularly, controlling uplink traffic in a wireless communication system.
2. Description of Related Art
Data transfer between mobile stations and base stations in General Packet Radio Service (GPRS) and Enhanced General Packet Radio Service (EGPRS) networks is accomplished using a stream of fixed-length, physical layer blocks. In addition to a number of payload bits, each block carries parity bits generated by a forward error correction code to protect the payload against transmission errors.
In EGPRS and GPRS networks, blocks are transferred over the air interface using unidirectional, physical layer connections known as Temporary Block Flows (TBFs). When the wireless data network wishes to transmit data packets to a mobile station, a downlink Temporary Block Flow is established. During downlink TBF establishment, the wireless data network assigns the mobile station a temporary address known as a Temporary Flow Indicator (TFI). The TFI is included in the header of each downlink block, thereby allowing mobile stations to easily determine which blocks are intended for it, and which are intended for other mobiles with active downlink TBFs. The TBFs are maintained only as long as there are blocks to be transferred. When the network has no more blocks to send to a mobile station, the downlink TBF is torn down, allowing the network to re-use the TFI for other mobiles.
Mobile stations wishing to transmit data packets to the network must first request establishment of an uplink TBF. If the network has sufficient uplink bandwidth to satisfy the request, the network establishes an uplink Temporary Block Flow, and tells the mobile which uplink timeslots it is permitted to use.
Uplink timeslots are shared among all mobiles with active uplink Temporary Block Flows. To coordinate uplink transmissions among mobiles with active uplink TBFs on the same timeslot, GPRS and EGPRS networks employ a three-bit long Uplink State Flag. During uplink TBF establishment, the network assigns each mobile an Uplink State Flag (USF) for each of its assigned timeslots. At any time, no two mobiles sharing the same uplink timeslot will have the same USF flag value. The network includes a USF flag in the header of each downlink block on a timeslot. When a mobile station detects its USF flag in a downlink block on one of its assigned timeslots, the mobile station begins transmitting uplink blocks on that timeslot during the following uplink block—roughly 20 ms after the mobile “sees” its USF flag in a downlink block. When a mobile's uplink TBF is torn down, the network is free to reassign the USF flags that were assigned to the mobile.
Data traffic surges in wireless data networks can strain the processing and buffering capacity of wireless data network equipment. An effective overload defense used to protect network resources during temporary and sustained overloads is for the network to drop new requests for Temporary Block Flows. Simply denying requests for new TBFs does not provide any defense against overloads caused by traffic carried on existing TBFs, however. And, because network resources have already been invested in setting up existing TBFs, and partially transmitting packets between the network and mobile stations, tearing down existing TBFs is an unattractive defense against overload. In addition, when TBFs are torn down, mobiles will retry shortly thereafter, thus, generating even more load.